In the operation of a pebble-bed HTR with multiple passes (like AVR or THTR), a certain proportion of the recirculated fuel elements (FE) must be removed from circulation to provide room for the addition of fresh fuel elements. It is thus naturally in the interest of good fissionable material economy to remove the fuel elements which have burned out to the greatest extent where possible. For this purpose each individual circulated fuel element is subjected to a measurement. What is measured is a physical parameter which constitutes a measurement of the degree of burn-out. It is important in such a system, in the interest of good measurement precision, not necessarily that there be a proportionality between this parameter and the degree of burn-out, but rather a greater measurement effectiveness and good reproducibility of the parameter which is measured. Based upon the measured parameter, a determination is made as to whether the fuel element is to be fed back to the reactor core and optionally to which zone of the reactor core it is to be fed, or whether it is to be removed.
Within the reactor core a fission process is carried out as a result of which fission products are produced by the fissionable material within the fuel elements. The individual fuel elements during the circulation are located outside the reactor core in the ball or pebble removal tube so that further fission processes are suppressed. The fission products within the fuel elements are however radioactive and emit on their part γ radiation (gamma radiation). For different fuel elements, the measured total γ radiation emitted from a fuel element under substantially identical conditions, for example the same duration after emergence of the fuel element from the reactor core, is correlated with its burn-out.
Up to now various measuring processes have been used to determine the degree of burn-out of ball-shaped fuel elements.
With the AVR (Working Group Test Reactor), because of the relatively small circulation velocity of the fuel elements of about 500 per day, a γ spectrometric measurement of the Cs137 present in the fuel element is possible with a liquid-nitrogen-cooled semiconductor detector. These measurements are only somewhat expensive and supply over acceptable measurement times of 20 to 40 seconds a measurement precision in the range of +2% for highly burned-out fuel elements.
With modern modular pebble-bed power reactors, like the HTR module of Siemens or the South African PBMR, the circulation speed is much higher by comparison with the AVR (about 4000 fuel elements per day) and the decay time of the fuel elements in the ball withdrawal tube is relatively short (about 2 days) so a direct translation of the measuring process from the AVR to the higher speed circulation of such reactors is not possible if only because of the short measuring time which is available. A shorter measurement time invariably gives rise to greater measurement error. Of greater significance is the fact that because of the very short decay time of the fuel elements, evaluation of the Cs137 line can be very imprecise. The high activity of the short-lived fission products is particularly detrimental as far as the γ measurement of the Cs137 is concerned since the evaluation of the typical 662 keV of the Cs137 is significantly influenced by the neighboring lines. Among these are the very strong 658 keV line of Nb97 (effective half life=16.8 hours), the weaker 661 keV line of Ba140 (half life 12.8 days) and the strong 668 keV line of the I132 (effective half life 76.3 hours). A corresponding correction of the measured Cs137 signal as a rule would require very expensive measurement technology to carry out. The fast circulation in combination with a short ball discharge tube and thus a short residence time in the ball discharge tube can thus give rise to a significant influence on the reproducibility of the Cs measurement. Concrete tests of an actual reactor are not however available as yet. Those skilled in the art have treated the attainable precision very differently. Generally however it has been believed that with highly burned-out fuel elements, it is not possible to do better than a mean measurement error of +10%.
In corresponding expert circles, alternatives have been proposed for the simple measurement of total γ activity of fuel elements for modern modular pebble-bed power reactors.
The γ activity of an irradiated fuel element is dominated in the reactor core and even after its emergence from the core in the case of a not too great decay time by the short-lived fission products. The contribution of the longer-life fission product to the intensity of the radiation is practically negligible. Fuel elements which have been burned out to a lesser extent have, in the reactor core and thus also shortly before their emergence from the core, a greater power production or power productivity than more burned-out fuel elements and thus also a higher (short lived) γ activity. The measurement effect in terms of the difference in γ radiation between a fuel element which has been burned out to a lesser extent and a fuel element which is highly burned out or burned out to a greater extent is very high. (In the case of the AVR with its comparatively long decay time of the fuel elements of an average of say one month, the γ activity of the fuel elements burned out to a lesser extent is always about 3 to 4 times higher than the γ activity of the highly burned-out fuel elements. These methods are indeed not very accurate although they are very easily carried out and unusually fast (measurement time about 1 second).
As state of the art, today the combination of measurements of the total γ activity and of the Cs137 radiation can be recognized. All of the fuel elements are thus subjected to a simple γ measurement (for example 1 second). Only with fuel elements which have been recognized as highly burned-out fuel elements is the γ activity value undertaken below the above-given limits as a parallel Cs137 measurement (about 10 seconds). Only after the evaluation of the Cs137 measurement is a determination made as to whether the fuel element is recirculated or withdrawn.
However, even with this combination method which permits the longer measurement duration for the Cs measurement as a rule, large mean errors have to be reckoned with which fuel elements which are burned out to a high extent. The experts in the field have indicated that the precision attainable is from +4% to +20%.